Advanced Vitreous State The Physical Properties of Glass Active - - PowerPoint PPT Presentation

Advanced Vitreous State – The Physical Properties of Glass

Active Optical Properties of Glass Lecture 1: Fluorescence, Amplifiers and Lasers

Denise Krol Department of Applied Science University of California, Davis Davis, CA 95616 dmkrol@ucdavis.edu

dmkrol@ucdavis.edu Advanced Vitreous State - The Properties of Glass: Active Optical Properties of Glass 1

Active Optical Properties of Glass

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• 1. Light emission

(fluorescence, luminescence)

Optical amplification and lasing

• 2. Nonlinear Optical Properties

Optical transitions, spontaneous emission, lifetime, line broadening, stimulated emission, population inversion, gain, amplification and lasing, laser materials, role of glass Fundamentals: nonlinear polarization, 2nd-order nonlinearities, 3rd-order nonlinearities Applications: thermal poling, nonlinear index, pulse broadening, stimulated Raman effect, multiphoton ionization

• Four things can happen when light proceeds into a solid.

Io IR

• Part of the light can be reflected by the

surface of the solid. Reflection

Io IT

• Part of the light can be transmitted

through the solid. Transmission

Io IA

• Part of the light can be absorbed by

coupling into the solid. Absorption

Io IS

• Part of the light can be scattered by the

atoms and defects in the solid. Scattering

• Therefore, for an incident beam of intensity Io entering the solid:

Io=IR+ IT + IA+IS

Optical properties of materials (Lucas, lecture 16)

Optical transitions: absorption and emission

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An atom/ion in a material, such as glass, can absorb light, hν = E2-E1 ν is the frequency of light (photon) E1 E2 After absorption, the material does not stay in the excited state indefinitely, but it will go back to the ground state either by emitting light

• r

giving off heat E1 E2 E1 E2 radiative decay spontaneous emission non-radiative decay

There is a certain probability (A21) for the atom to decay radiatively

Spontaneous emission and lifetime

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E1 E2 A21

dN2 dt = A21N2 N2 = N20e

A21t = N20et /

For a collection of N2 atoms in the excited state: With solution:

τ is the lifetime

Including non-radiative decay: Atot = A21 + Anr τ = 1/Atot

Line broadening: homogeneous

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E1 E2 ν0=E2-E1/h

The finite lifetime of the excited state leads to a broadening of the emission linewidth:

I

( ) = I0

Atot /4 2

( )

2 + Atot /4

( )

2

Δν=Atot/2π

ν-ν0 Iem

The lineshape is Lorentzian and the same for all atoms homogeneous broadening

There is also a broadening that results from the fact that not all atoms have the same surroundings (glass!) different atoms have slightly different transition frequencies The spread in frequencies is characterized by ΔνINH

Line broadening: inhomogeneous

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I

( ) = I0

2 ln2

( )

1/ 2

1/ 2 INH exp 4 ln2

( ) 0 ( )

2

INH

( )

2

• The resulting lineshape is Gaussian inhomogeneous broadening

Iem ν−ν0 ΔνINH

Line broadening: homogeneous vs inhomogeneous

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Lorentzian Gaussian Iem ν−ν0

curves have same area and half-width

In general, one or the other line broadening mechanism can dominate In glasses -due to their disorder- inhomogeneous broadening almost always dominates

Glass: strong inhomogeneous broadening

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From W.T. Silfvast, Laser Fundamentals, 2nd ed., Cambridge (2004)

E1 E2 E1 E2 photon

+ + stimulated emission

Spontaneous vs stimulated emission

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ν21=E2-E1/h E1 E2 photon E1 E2

+

E1 E2

spontaneous emission light amplification!

Stimulated emission vs absorption

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E1 E2 E1 E2 photon +

+ stimulated emission But we also have absorption

E1 E2 photon + E1 E2

To have net amplification of light (gain) we need N2 > N1 We need population inversion

Population Inversion

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If there are only 2 levels inversion is not possible But if we have >3 levels inversion can be obtained Example: E0 E3 E1 E2

pump transition amplification (or laser) transition fast relaxation fast relaxation

Optical Amplification and Lasing

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gain medium

pump ν21 amplifier laser = amplifier + optical cavity pump

gain medium

R=100% R=90%

Because of stimulated process, amplified light has direction and phase of incoming signal

laser light has the following properties:

• highly directional
• highly monochromatic
• highly coherent

Optical cavity modes

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cavity L axial mode frequencies ν = nc/2L axial mode separation Δν=c/2L

3-Level vs 4-Level Laser System

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E0 E3 E1 E2

pump transition laser transition

E3 E1 E2

pump transition laser transition In 3-level system more than 50% of level 1 needs to be pumped, so it is harder to obtain inversion: Pump and laser transition share a level 3-level system example : Er3+ 4-level system example : Nd3+

Some simple laser equations

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e2gthL =

1 R 2 gain =loss and ν = nc/2L gth = σΔNth where σ is the emission cross-section (m2). R = mirror reflectivity note: cw lasers! The lasing threshold is achieved when the pump rate (proportional to pump power) is high enough to obtain ΔNth. If the pump rate is increased further the steady state laser intensity (power/area), Iss, grows according to Iss = (P/Pth -1)Isat Here P is the pump power, Pth the pump power needed to reach threshold and Isat the saturation intensity (a fixed parameters for a given laser transition)

What makes a good laser transition/material?

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For cw operation:

• Good pumping efficiency
• Purely radiative lasing transition
• Small difference between pump and laser wavelength
• Fast relaxation from 3 ->2 and 1->0

E0 E3 E1 E2

Other important materials properties:

• Thermal conductivity
• Optical quality
• Mechanical properties

Host material influences emission and laser characteristics Glass is used as a host material rare-earth ions (f-shell, narrow transitions) Nd3+ Er3+ Yb3+

Solid-state laser materials and glass

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Most solid state laser materials fall in one of 2 categories:

• 1. Dielectric materials (host material)doped with active ions: Nd3+:YAG,

Cr3+:Al2O3

• 2. Semiconductor materials: GaAs, GaN

transition-metal ions (d-shell, broad transitions) Cr3+ Ti3+

Nd-doped glass amplifiers

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Nd3+ energy level diagram corresponding 4-level laser diagram for 1064 nm transition

crystals: narrow lines, better thermal conductivity glass: uniform & large pieces, broader lines, lower thermal conductivity

lasers amplifiers

Nd-doped glass amplifiers

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From W. Koechner, Solid State Laser Engineering

Phosphate glass preferred

Erbium Doped Fiber Amplifier: EDFA

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4I13/2

980 nm 1520-1560 nm

4I11/2 4I15/2

1480 nm Energy levels of Er3+ Pumping bands @ 980 or 1480 nm Schematic diagram of EDFA Glass fibers: long interaction lengths, compact and robust

Yb: Glass Fiber Laser

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Absorption and emission cross sections of ytterbium-doped germanosilicate glass, as used in the cores of ytterbium-doped fibers. (Data from spectroscopic measurements by R. Paschotta)

Very small difference between pumping and lasing wavelengths leads to minimal heating Only 2 levels: no excited state absorption Very high powers can be achieved: 50 kW!! Diode pumping Fairly large bandwidth: Δν ~ 1/Δτ -> short pulse operation Quasi 3-level laser